Highway maintenance vehicles are often parked near roadways or move slowly in or adjacent to high-speed traffic lanes. In these situations, maintenance vehicles pose a serious hazard to the motoring public and the workers. The safety risk associated with these work vehicles is normally minimized through the use of a truck mounted attenuator (TMA).
A TMA is essentially a crash cushion that is cantilevered off of the back of the support truck. The combination of a cantilevered mounting structure and the length of most TMAs produce the potential for large oscillations in the vertical position of the attenuator as the support truck moves down the highway. In other words, the attenuator bounces up and down. These oscillations produce two problems, fatigue loading of TMA components and the potential for the end of the TMA to strike the roadway surface. Support elements and every connection in the structure of a conventional TMA must be strengthened to resist the long term fatigue loading induced by the vibratory oscillations in the unit. Further, in order to prevent large oscillations that could allow the back of the TMA to strike the pavement, support trucks must be driven at a very low speed when the TMA is in use. This speed limitation forces the TMA to be mounted to the support truck with a lifting system that raises the unit to a near vertical position when it is no longer needed.
The support structures and lifting systems used with most TMAs must be securely attached to the work vehicle, thereby preventing the unit from being easily detached from the truck. As a result, TMA support trucks often become dedicated vehicles that can only be used as shadow vehicles to protect motorists from roadway maintenance activities.